Abstract
The surface anchoring energy of the nematic liquid crystal 4-octyl-4ʹ-cyanobiphenyl (8CB) has been deduced from the transient shear stress response, making use of the fact that shearing a tumbling nematic with strong anchoring on the shearing plate generates elastic forces that push the director toward the vorticity axis in the bulk. This feature is manifested in 8CB near the nematic–smectic transition by a long-term decay in the shear viscosity, whose rate depends on the anchoring energy, As . The calculations have been performed for the natural boundary conditions for the director associated with the stationary states of the total free energy, including a surface term proportional to As . It has been found that As ≈ 3.3 × 10−6 J m−2 for metallic plates covered with lecithin, in agreement with data obtained by the classical method for glass substrates covered with lecithin.
Acknowledgements
This study was partly supported by Fundação para a Ciência e a Tecnologia (Portugal) under the research contract C2007–443–CENIMAT–1 with A. Véron.
Notes
1. The Leslie viscosities satisfy the Parodi relation α2 + α3 = α6 − α5.
2. For aligning nematics a difference would occur only in a thin layer close to the plate and may be considered as unimportant for bulk properties.
3. By Neumann-like boundary conditions we mean a relation between the normal gradients of the director and the director on the plate.
4. and
denotes the part of the shear stress components
and
that depends only on the molecular field.